1. Field of the Invention
This invention relates to a turbine and a turbocharger using the same and, more particularly, to a turbine provided with a rotor which is driven into rotation by a working fluid ejected from a nozzle and which may be used as a small-sized steam turbine, gas turbine or a turbocharger.
2. Description of the Prior Art
A turbine is constructed in general by a casing and a rotor rotatably carried in the casing and provided with a large number of blades on the circumference thereof, and is adapted for driving the rotor into a high-speed rotation by laterally discharging a gas at a high speed towards the blades from a nozzle provided on the casing. Each blade of the turbine is constituted by a concave surface generating a positive torque and a surface generating a negative torque so that a torque is produced which is the result of counterbalancing of the two torques.
Hence, with such conventional turbine, for producing a low-speed high-torque output, a rotor fitted with blades each having as large an outside radius as possible is set into a high-speed rotation and decelerated by a speed-reducing unit for producing a large rotational force, despite the fact that the problem is raised in connection with strength. Such conventional turbine is larger in size, while requiring a number of auxiliary devices, so that it tends to be expensive.
Thus a sufficiently high rotational force cannot be developed with the above described conventional turbine by simply reducing the size of the turbine and thereby reducing the costs. Besides, the space between the casing and the blades unavoidably leads to leakage of the unused working fluid and renders it difficult to raise the rotational force.
For improving the above described conventional turbine, a turbine has been proposed in the U.S. Pat. No. 4773818 in which a spiral flow of the working fluid is generated by a casing having a spirally extending groove on its inner periphery and a rotor having a spirally extending groove on its cuter periphery, and in which blades are provided at a predetermined interval within the spiral groove of the rotor.
With this improved type of the turbine, a low-speed high-torque output may be developed despite its small size. However, since the groove is formed on the inner peripheral surface within the casing, the working fluid, such as the steam, tends to leak through the spiral groove without contributing to the rotor revolutions, thus lowering the operating efficiency. In addition, the higher the number of revolutions of the rotor, the more the amount of the working fluid flowing through the spiral groove, due to the effect of a centrifugal force, thus lowering the turbine efficiency. Moreover, when the working fluid flows in the groove on the inner periphery of the casing, especially when it flows as it is forced towards the groove bottom under the effect of a centrifugal force, frictional losses are increased, thus further lowering the turbine efficiency.